Because of its high conductivity and low cost, copper is an ideal metal for the internal electrodes in multilayer ceramic capacitors (MLCs). However, the use of copper electrodes in MLCs has been held back because of the following technical difficulties:
(a) the problem of reproducibly and reliably processing multilayer ceramic devices with many layers of copper electrodes and
(b) the problem of achieving high dielectric constants for ceramic compositions which must sinter below the melting point of copper (1083.degree. C.) and which also must be resistant to reduction in the atmospheres of low oxygen content that are needed to protect copper from oxidation when the multilayer ceramic/metal structure is fired.
As described in our co-pending U.S. patent application Ser. No. 003,259 filed Jan. 13, 1987, the processing of MLCs with many copper electrodes (e.g., 25) can be accomplished by the use of an electrode paste made with an appropriate acrylic binder and by firing the capacitors in an atmosphere which protects the copper from oxidation during firing, e.g., a mixture of nitrogen, carbon dioxide and hydrogen. Furthermore, ceramic compositions with dielectric constants (K) greater than 1000 suitable for use in MLCs with copper electrodes are described in U.S. Pat. No. 4,101,952. These compositions are based on alkaline earth titanates with additions of alkaline earth aluminoborate glass to achieve sintering at 900.degree.-1080.degree. C. The compositions are free from readily reducible oxides such as lead oxide, bismuth oxide and cadmium oxide and from volatile fluorides as well. Thus, they are chemically stable during the sintering process needed for MLCs with copper electrodes. However, the dielectric constant of these compositions is only as high as 4000, which is too low to provide adequate capacitance in the capacitors of small size normally used for decoupling and by-pass applications and meeting the A.E.I. specifications on temperature stability of Y5V or Z5U. Accordingly, there exists a substantial need for dielectric compositions which (1) are chemically stable and resistant to reduction, (2) can be sintered to a dense hermetic structure below the melting point of copper, and (3) have dielectric constants significantly higher than those which were achieved heretofore.